As a result of economic forces, environmental considerations, waste disposal regulations and other factors, activities in the fields of thermal and chemical process engineering have gravitated toward the use of microreactors for research and development, including modeling studies and chemical reactions. In addition, microreactors are finding application in pharmaceutical and biological research, development and analysis. A microreactor is a device that enables chemical reactions, either gaseous or liquid, to be done on the low milliliter scale (510 ml) as opposed to earlier laboratory “bench top” or pilot plant scales that varied in size from many tens of milliliters to liters in the former and up to a hundred liters, or more, in the latter. The microreactor is generally a continuous flow reactor that brings the reaction components together in a small reactor channel. FIG. 1 is a top view illustrating one of the simplest designs, a “T-shaped” microreactor 10. In a typical reactor of this design a T-shape is etched into a plate 20 to a selected depth (for example, 50 μm deep by 100 μm wide) and the etched plate is then covered with another plate (14 in FIG. 2) so that the etched portion forms an enclosed channel. The cover plate has openings (three illustrated in FIG. 1) so that fluids (gaseous or liquid) can be added and removed from the reactor. A reaction is be carried out by pumping a first fluid containing a first reactant through opening 22 and a second fluid containing a second reactant through opening 24. The fluids are pumped at the same rate so that they meet at the position 26, the top of the vertical part 28 of the T where they begin to mix and react as they proceed (illustrated by the broad arrow 27) down the vertical part 28 of the T. The reaction product is removed at the opening 30. FIG. 2 is a side view illustrating etched plate 20, top plate 14, openings 22, 24 and 30, and fluid illustrated as light grey in the reactor. The dashed line 16 illustrates the junction of placed 14 and 20.
While the simple design illustrated in FIG. 1 is satisfactory for some reactions, for others a more complex design is required. For example, it may be desirous to add mixing baffles; openings for the further addition of reactants as the fluids travel from the beginning to the end of the reactor; space for heating and/or cooling elements with their associated connections; thermocouples and their connections; and other elements as may be need to carry out, control or monitor the reactions that occur within the microreactor. As a result the design of the reactor can become quite complicated; which in turn means that the construction of the reactor itself becomes complicated and expensive if etching techniques are used to construct parts of the microreactor. In addition, while materials such as metals, silicon and certain polymers can be used to fabricate microreactors, these materials are not well suited for chemical reactions at high temperature and/or that use corrosive reactants. As a result of the foregoing problems, a simplified method for making microreactors is desirous; and it is further desired that such reactors be made of glass or ceramic materials due to their high thermal stability and their chemical durability and/or inertness to the vast majority of chemicals and solvents.
As a result of the foregoing problems, methods of making microreactors using “frits”, particularly glass fits, have been developed. A frit is a powdered glass that sinters to form a structure that incorporates, for example, microreactor features and/or elements. To make the microreactor the fit is typically sandwiched between two substrate layers that may themselves incorporate some microreactor elements such as the openings for reactant(s) entry and exit, control leads for heaters and other elements, some of which have been described above. The resulting “sandwiched” microreactor must be “fluid tight” so that reactants and/or solvents do not escape. Commonly owned U.S. Patent Application Publication No. 2004/0152580 A1 (assigned to Corning Incorporated) describes borosilicate glass compositions and their use to make microfluidic devices such as the microreactors described above. As mentioned in U.S. 2004/0152580 A1, the problem with PYREX® glass flits is that they undergo devitrification (that is, crystals of different materials are formed) during sintering at temperatures in the range of 700-800° C. However, there is a lowering of mechanical strength due to both the formation of crystals with a high coefficient of thermal expansion and the volume change that is associated with the phase transformation of cristobalite crystals at approximately 200° C. This can lead to frit cracking on cooling after sintering. As a result, the inventors in U.S. 2004/0152580 A1 proposed that alumina be added to the borosilicate glass composition. The addition of alumina causes the sintering ability of the fit to decrease and reduces the fluidity of the fit. While the materials describes in U.S. 2004/0152580 A1 resulted in an improved fit material, further improvements are needed to both frit compositions and to the method of making fits that can be used in microreactors. The present invention is directed to improved compositions that can be used to make glass flits that can be used in microreactor and the methods of making such flits.